U.S. Pat. Nos. 4,859,858 and 4,859,859, both entitled GAS ANALYZERS, were issued to Knodle et al. on Aug. 2, 1989. Both patents disclose state-of-the art apparatus for outputting a signal indicative of the concentration of a designated gas in a sample being monitored by the apparatus. These patents are hereby incorporated by reference thereto into this application.
The gas analyzers disclosed in the '858 and '859 patents are of the non-dispersive type. They operate on the premise that the concentration of a designated gas can be measured by: (1) passing a beam of infrared radiation through the gas, and (2) then ascertaining the attenuated level of the energy in a narrow band absorbable by the designated gas. This is done with a detector capable of generating a concentration proportional electrical output signal.
One important application of the invention at the present time is in capnometers for monitoring the level of carbon dioxide in the breath of a medical patient. This is typically done during a surgical procedure as an indication to the anesthesiologist of the patient! s condition. As the patient's well being, and even his life, is at stake, it is of paramount importance that the carbon dioxide concentration be measured with great accuracy.
In a typical instrument or system employing non-dispersive infrared radiation to measure gas concentration, including those disclosed in the '858 and '359 patents, the infrared radiation is emitted from a source and focused by a mirror on the gases being analyzed. After passing through the body of gases, the beam of infrared radiation passes through a filter. That filter absorbs all of the radiation except for that in a narrow band centered on a frequency which is absorbed by the gas of concern. This narrow band of radiation is transmitted to a detector which is capable of producing an electrical output signal proportional in magnitude to the magnitude of the infrared radiation impinging upon it. Thus, the radiation in the band passed by the filter is attenuated to an extent which is proportional to the concentration of the designated gas. The strength of the signal generated by the detector is consequently inversely proportional to the concentration of the designated gas and can be inverted to provide a signal indicative of that concentration.
While a non-dispersive analyzer must be tailored to the specific gas of interest, it is typically small, relatively cheap, and rugged enough to be used in medical and other demanding environments.
Of considerable, if not critical, importance in apparatus and systems of the character just described is the source or emitter which produces the beam of infrared radiation. In those non-dispersive gas analyzers disclosed in the '858 and '859 patents, the emitter has a substrate of a material with low thermal conductivity such as steatite. Two T-shaped conductors or terminals are bonded to the upper surface of the substrate in spaced relationship; and a film of an emissive, electrically resistive material is superimposed on the conductors and bonded to the upper surface of the substrate with its ends overlapping and electrically connected to the conductors. This emitter is attached to posts at its opposite ends and supported by those posts from a metallic emitter mount with the emissive film facing the mount. That component has a polished, parabolic, mirror surface formed in the surface which the emitter faces. This mirror collimates the emitted infrared radiation and focuses the collimated radiation into a beam directed along the optical path of the device or system in which the infrared radiation source is employed.
In the previously disclosed infrared radiation sources, the substrate bearing the emissive film was fixed at both of its ends to the supporting posts. As the substrate was heated by the emissive film, it grew or increased in length due to thermal expansion. This has led to failure of the patented type of infrared radiation source because of the stresses that were consequently imposed on the substrate and substrate-supported components.
Also, assembly of the patented infrared radiation sources is somewhat complicated and exacting. For these and other reasons, the patented sources may prove to be somewhat less than optimal in applications where a high quality, yet inexpensive, infrared radiation source is required.